Thanks, that seems to work in the UI. I’ll try it with the API tomorrow.
In case anyone else is looking for it in the future:
PUT https://forum.example.com/topics/bulk
Form data copied from the browser console:
topic_ids%5B%5D=12345&operation%5Btype%5D=append_tags&operation%5Btags%5D%5B%5D=tagname
Decoded:
topic_ids[]=12345&operation[type]=append_tags&operation[tags][]=tagname
Update: it worked with the API too.